Simultaneous and selective partitioning of via structures using plating resist

ABSTRACT

Systems and methods for simultaneously partitioning a plurality of via structures into electrically isolated portions by using plating resist within a PCB stackup are disclosed. Such via structures are made by selectively depositing plating resist in one or more locations in a sub-composite structure. A plurality of sub-composite structures with plating resist deposited in varying locations are laminated to form a PCB stackup of a desired PCB design. Through-holes are drilled through the PCB stackup through conductive layers, dielectric layers and through the plating resist. Thus, the PCB panel has multiple through-holes that can then be plated simultaneously by placing the PCB panel into a seed bath, followed by immersion in an electroless copper bath. Such partitioned vias increase wiring density and limit stub formation in via structures. Such partitioned vias allow a plurality of electrical signals to traverse each electrically isolated portion without interference from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/369,448, filed Mar. 6, 2006, which claims the benefit of ProvisionalU.S. Patent Application No. 60/658,886, filed Mar. 4, 2005, both ofwhich are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to printed circuit boards (PCBs), and moreparticularly, to systems and methods for simultaneously partitioning avia structure into electrically isolated portions by using platingresist within a PCB stackup for allowing a plurality of electricalsignals to traverse each electrically isolated portion withoutinterference from each other.

BACKGROUND

Consumers are increasingly demanding both faster and smaller electronicproducts. The use of PCBs has grown enormously as new electronicapplications are marketed. A PCB is formed by laminating a plurality ofconducting layers with one or more nonconducting layers. As the size ofa PCB shrinks, the relative complexity of its electricalinterconnections grows.

A plated via structure is traditionally used to allow signals to travelbetween layers of a PCB. The plated via structure is a plated holewithin the PCB that acts as a medium for the transmission of anelectrical signal. For example, an electrical signal may travel througha trace on one layer of the PCB, through the plated via structure'sconductive material, and then into a second trace on a different layerof the PCB.

Unfortunately, due to limitations within the prior art, the plated viastructure may be longer than necessary to perform the function ofelectrical connectivity. For example, the plated via structure mayextend completely through the PCB but only connect two traces on twoproximate adjacent layers. As a result, one or more stubs may be formed.A stub is excessive conductive material within the plated via structurewhich is not necessary to transport the electrical signal.

When a high speed signal is transmitted through the plated viastructure, a “stub effect” may distort the signal. The stub effect is aresult of the useless excess conductive material present within theplated via structure. The stub effect occurs when a portion of thesignal is diverted away from the trace connections and into one or morestubs of the plated via structure. The portion of the signal may bereflected from the end of the stub back toward the trace connectionsafter some delay. This delayed reflection may interfere with signalintegrity and increase, for example, the bit error rate of the signal.The degenerating effect of the stub effect may increase with the lengthof the stub. As much as 50% of signal attenuation at signals running at10 Gigabits per second may be due to the stub in the plated viastructure. Via structures with short stubs can be manufactured butrequire sequential processing, which increases costs substantially.

FIG. 1 is an illustration of a PCB 100 with a plated via structure 110and a stub 170 in the prior art. The PCB 100 consists of conductinglayers 130 separated by nonconductive dielectric layers 120. Typically,the plated via structure 110 includes a barrel (i.e., shaft of the viastructure) that is cylindrical in shape and is plated with a conductivematerial 180. The plated via structure 110 allows an electrical signal160 to transmit from a trace 140 on a first conducting layer 130 of thePCB 100 to a trace 150 on a second conducting layer 130. The stub 170 ofthe plated via structure 110 is the unnecessary portion of the platedvia structure 110, which may create the stub effect.

FIG. 2 is an illustration of the PCB 100 with the plated via structure110 after the stub 170 (shown in FIG. 1) has been removed bybackdrilling in the prior art. Backdrilling the unnecessary portion ofthe plated via structure 110 to reduce or remove the stub 170 is onemethod to reduce the stub effect. Backdrilling is a viable alternativeto sequential layer processing but has limitations. Typically, a drillbit backdrills the stub 170 thereby removing a portion of theunnecessary excess conductive material of the plated via structure 110.A backdrilled hole 200 is created once the drill bit removes a portionof the stub 170 from the plated via structure 110. The drill bit iscommonly a carbide drill bit in a computer numerically controlled (CNC)drill machine. As a result of backdrilling, the portion of the stub 170of the plated via structure 110 is removed, thereby reducing, but notcompletely eliminating, parasitic capacitance, parasitic inductance, andtime delay, which may interfere with signal integrity.

In most cases, design concessions need to be made to allow fordeviations in the accuracy of the drilling equipment. If thebackdrilling is inaccurate (e.g. too deep or off center), then afunctional portion of the plated via structure 110 may be removed andthe PCB 100 may be ruined. As a consequence, a new PCB 100 must bereconstructed and backdrilled. Thus, yields are reduced and costs areincreased.

The backdrilling process is also limited in the tolerances that can bereliably held. Backdrilling is typically only controllable to a depthtolerance of +/−5 mils. In many cases, further design concessions needto be made due to limitations in the strength and consistency of thelayers to allow for variations in the placement, width, and direction ofdrilling.

Yet another limitation is that many designs require the backdrilling ofmultiple plated via structures 110 where the stubs 170 may be atdifferent depths. This requires specialized programming of the drilltool files, which takes time and money to produce.

Further, backdrilling multiple plated via structures 110 typically is aserial process, so that the time needed to backdrill the PCB 100increases with the number of stubs 170. If any one of the stubs 170 isdrilled improperly, the PCB 100 may be ruined. Therefore, backdrilling anumber of stubs 170 increases the probability of damage to the PCB 100.

Another limitation is that many designs also require stubs to be removedfrom both surfaces of the PCB 100. This requires that the PCB 100 bereoriented during the backdrilling process, which further takes time,requires additional programming, and adds potential error to theaccuracy of the backdrilling process.

Further, drill bits are prone to breakage which reduces yields andrequires rework of the PCB 100. The process of reworking each individualplated via structure 110 adds cycle time and increases costs inproduction. Moreover, drill bits are expensive, which further drives upcosts.

One consequence of backdrilling is that the volume of the removed stubbarrel is not functional in the context of circuit routing. No othertrace or interconnect on any layer can pass through the volume of theremoved stub. Circuit traces need to be re-routed around such volumes.In most cases, additional layers need to be added to effectively routeall the traces in a given design and thus add to complexity and cost.

PCBs can be split into two or more sections to reduce stub lengths orincrease wiring density using methods known in the art such assequential processing techniques. With sequential processing, twoseparate PCB subassemblies are individually manufactured. The twosubassemblies are subsequently laminated together and through-holes orvias are plated to connect the two individual PCBs into one. Stubs canbe controlled in this manner, but are limited to the layers between thetwo individual sub assemblies. Because of the “sequential nature” ofsuch a lamination process, additional process steps are required andcost and cycle time to manufacture is significantly increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a PCB with a plated via structure and astub in the prior art.

FIG. 2 is an illustration of the PCB with the plated via structure afterthe stub has been removed by backdrilling in the prior art.

FIG. 3 is an illustration depicting a PCB with a plated via structureformed through a plating resist, according to certain embodiments.

FIG. 4 is an illustration depicting a core sub-composite structurecovered with a layer of etch resist that is selectively exposed toelectromagnetic radiation, according to certain embodiments.

FIG. 5 is an illustration depicting the conducting layers and thedielectric layer of sub-composite structure with an area of altered etchresist, according to certain embodiments of the invention.

FIG. 6 is an illustration depicting the conducting layers and thedielectric layer of a sub-composite structure with the altered etchresist and a portion of the conducting layer removed to form a clearancein the conductive layer, according to certain embodiments.

FIG. 7 is an illustration depicting the conducting layers and thedielectric layer of sub-composite structure with the unaltered etchresist removed, according to certain embodiments.

FIG. 8 is an illustration depicting the conducting layers and thedielectric layer of a sub-composite structure with the plating resistdeposited within the clearance, according to certain embodiments.

FIG. 9 is an illustration depicting a PCB stackup with a partitionedplated via structure formed using a thicker layer of plating resist,according to certain embodiments.

FIG. 10 is an illustration depicting a PCB stackup with a partitionedplated via structure formed by selectively depositing plating resist ina clearance formed in a conducting layer and an adjacent dielectriclayer of a sub-composite structure, according to certain embodiments.

FIG. 11 is an illustration depicting a PCB stackup with a partitionedplated via structure formed by selectively depositing plating resist onthe surface of a sub-composite structure on an anti-pad region of thesurface that is coplanar with the top conducting layer of thesub-composite structure, according to certain embodiments.

FIG. 12 is an illustration depicting a PCB stackup with a partitionedplated via structure formed by selectively depositing plating resist ona conductive region or conductive pad on the surface of thesub-composite structure, according to certain embodiments.

DETAILED DESCRIPTION OF THE INVENTION

A cost effective and efficient system to minimize signal degradation isto electrically isolate, reduce, or eliminate a stub by controlling theformation of a conductive material within a plated via structure of aprinted circuit board (PCB). One or more areas of plating resists withinthe via structure are used to resist the formation of conductivematerial by intentionally creating one or more voids in the viastructure. As a result, the formation of conductive material within thevia structure may be limited to those areas necessary for thetransmission of electrical signals. According to certain embodiments,the partitioning of the via structure into electrically isolatedsegments can dramatically increase the route capabilities or wiringdensity of a PCB design. This is because each electrically isolatedsegment of the partitioned via can be used to electrically connectsignals on layers associated that particular segment.

A multilayer PCB can be a chip substrate, a motherboard, a backplane, abackpanel, a centerplane, a flex or rigid flex circuit. The invention isnot restricted to use in PCBs. A via structure can be a platedthrough-hole used for transmitting electrical signals from oneconducting layer to another. A plated via structure can also be acomponent mounting hole for electrically connecting an electricalcomponent to other electrical components on the PCB.

The methods to electrically isolate, reduce, or eliminate a stub withinvia structures of PCBs may be faster and more efficient thanbackdrilling. Plating resists may be placed within the many clearancesin the conducting and/or dielectric layers of the PCB simultaneously. Inmost cases PCBs can have through-holes and vias in the order of 100,000plus. At the same time, the multi-layer PCB can have multiple layers. Itwould be advantageous to partition each of the vias and control the stubto variable degrees for each via. In other words, each via can bepartitioned at different layers and at different locations. To be ableto partition all the vias simultaneous on a single panel, plating resistcan be selectively deposited on a selected layer of each sub-compositecore during the making of the PCB stackup prior to drilling andsubsequent plating of the vias in the panel. For example, all theclearances within a layer of a PCB may be formed concurrently. Inanother example, the conductive material may be formed within all of thevia structures of a PCB at the same time. In contrast, as discussedpreviously, backdrilling is generally performed upon one via structureat a time. Thus, methods incorporating plating resists to limit stubformation may allow for faster production of PCBs than backdrilling.

FIG. 3 is an illustration depicting a PCB 300 with a plated viastructure 330 formed through a plating resist 370, according to certainembodiments. The PCB 300 includes conducting layers 310 a-310 eseparated by dielectric layers 320 a-320 e. The plated via structure 330is plated with a seed conductive material 390 and a further coating ofconductive material 392. The plated via 330 is effectively partitionedinto a plurality of electrically isolated portions (330 a, and 330 b) byselectively depositing plating resist in a sub-composite structure formaking the PCB stackup. A method of partitioning a plated via such asplated via 330 is described herein with reference to FIGS. 4 to 8.

FIG. 3 shows that the plated via allows an electrical signal 360 totransmit from one trace 340 or component mounting pad on a firstconducting layer 310 a to another trace 350 on a second conducting layer310 b of the PCB 300 by traversing the isolated portion 330 a of the via330. Similarly, the isolated portion 330 b of the via 330 allows anotherelectrical signal 362 to transmit to trace 380 without interfering withthe signal 360.

Plating resist is a generally nonconductive material deposited in one ormore clearances of conducting and dielectric layers. For example, inFIG. 3, plating resist is deposited in a clearance in the conductinglayer 310 d. When PCB 300 is placed in a seed or catalyzing bath, theseed will deposit on all areas of the via wall but will not deposit onthe plating resist. Should small amounts of seed be deposited on theplating resist, a post processing operation can be utilized to removethese residual deposits. Subsequently, when the panel is placed into anelectroless copper or electrolytic copper plating bath, copper willplate where there is seed or conductivity and will not plate or depositin the area where there is plating resist. The plating resist will forma cylindrical void that effectively partitions the barrel of the viainto segments.

The plating resist 370 prevents the deposition of the catalyzingmaterial 390 and conductive material 392 within the via structure 330 atthe conducting layer 310 d. As a result, via 330 is partitioned into theelectrically isolated portions 330 a, and 330 b. Consequently, theelectric signal 360 travels from the first conducting layer 310 a to thesecond conducting layer 310 b without signal integrity being degradedthrough interference caused by section 330 b. The conductive material392 of the plated via structure 330 is the medium through which theelectrical signal 360 travels from the first conducting layer 310 a ofthe PCB 300 to the second conducting layer 310 b. Similarly, electricsignal 362 traverses plated via 330 conductive layer 310 e. The platedvia structure 330 may be of any shape.

Some examples of the conductive or catalytic material 390 areelectroless copper, palladium seed. The catalytic seeding process canalso include electrophoretic plating, or direct metallization. Theplating process wherein the conductive material 392 such as conductivemetal, or copper is deposited within the via structure 330 may compriseelectrolytic plating, or an electroless process.

The PCB 300 can have any number of conducting layers and dielectriclayers. FIG. 3 only shows five layers of conducting layers 310 a-310 eand five layers of dielectric layers 320 a-320 e for the sake ofsimplicity. Each of the conducting layers 310 a-310 e may comprise apartial or full layer such as a power or ground layer, may comprise alayer of circuit traces, or may comprise a layer with both circuittraces and a partial layer such as a ground layer. A non-limitingexample of the conducting layers 310 a-310 e is copper. Somenon-limiting examples of dielectric layers 320 a-320 e are FR-4, epoxyglass, polyimide glass, ceramic hydrocarbon, polyimide film, resinimpregnated woven glass, film, resin impregnated matte material, Kevlar,paper, and resin dielectrics with dispersed nano-powders. According tocertain embodiments, the partitioned via is filled with an insulating orresistive paste to improve reliability or functionality.

A method of partitioning a plated via such as plated via 330 isdescribed herein with reference to FIGS. 4 to 8. As described furtherherein, a clearance is a hole that is within at least one conductinglayer 310 a-310 e and/or at least one dielectric layer 320 a-320 e. Forexample, a clearance may be formed in the conducting layer 310 e. Eachclearance has a radius greater than the plated via structure 330. Theformation of the clearances through an etching process is describedbelow with reference to FIGS. 4-8.

FIGS. 4-8 are examples depicting etching a clearance within theconducting layer 310 d, as well as the placement and deposition of theplating resist 370, according to certain embodiments of the invention.It is to be noted that the etching as described with reference to FIGS.4-8 would apply to both conductive layers of the sub-compositestructure. For purposes of simplicity, the etching is described withreference to one conductive layer (310 d) of FIG. 4-8. Further, forsimplicity, FIGS. 4-8 describes the selective deposition of platingresist on one location in a core sub-composite structure. However, it isunderstood that plating resist can be selectively deposited on multiplelocations in the sub-composite structure depending on the PCB design.Moreover, each sub-composite structure may have the plating resistselectively deposited on different layers than that of othersub-composite structures so as to achieve the desired PCB design bylaminating these various sub-composite structures to form the PCBstackup.

FIG. 4 is an illustration depicting a core sub-composite structurecovered with a layer of etch resist that is selectively exposed toelectromagnetic radiation, according to certain embodiments. FIG. 4shows sub-composite structure 402 (also referred to herein as a core)that includes a dielectric layer 320 d sandwiched between two conductinglayers 310 d, and 310 e. Conductive layer 310 d is covered with an etchresist 400. Portions of the etch resist is covered with a mask 410.

The etch resist 400 is any material that is applied to an area of theconducting layer 310 d to prevent reaction of that area during anelectromagnetic, chemical, or electrochemical etching process. The etchresist 400 may be processed by a lithographic process, by selectivedeposition, or by direct laser imaging. Some examples of etch resist 400are photoresist, organic material, dry film, sheet, paste, polymer thickfilm, and liquid.

Mask 410 is a film or plate that selectively covers an area to preventreaction of the covered area during the electromagnetic, chemical, orelectrochemical reaction. Some examples of the mask 410 are silver film,glass, or diazo film. Mask 410 may be positioned over the etch resist400 with a mask aligner (not depicted) which is configured to controlthe placement of the mask 410. The exposed portion of the etch resist400 is exposed to electromagnetic radiation 420, or a laser, asnon-limiting examples, and altered to make the exposed etch resistremovable while leaving the covered etch resist undisturbed. In the caseof using a laser, mask 410 is not needed.

FIG. 5 is an illustration depicting the conducting layers 310 d, 310 eand the dielectric layer 320 d of sub-composite structure 402 with anarea of altered etch resist 500, according to certain embodiments of theinvention. The electromagnetic radiation 420 (FIG. 4) has beenterminated and the mask 410 (FIG. 4) has been removed thereby exposingthe unaltered etch resist 400.

FIG. 6 is an illustration depicting the conducting layers 310 d, 310 eand the dielectric layer 320 b of sub-composite structure 402 with thealtered etch resist 500 (FIG. 5) and a portion of the conducting layer310 d removed to form a clearance 600 in the conducting layer 310 d,according to certain embodiments. The altered etch resist 500 (FIG. 5)has been removed by methods well known in the art thereby exposing aportion of the conducting layer 310 d. The exposed portion of theconducting layer 310 d is then etched to form the clearance 600 andexpose the dielectric layer 320 d. Clearance 600 can be in a ground orpower plane or in conductive pad or feature on a signal layer.

FIG. 7 is an illustration depicting the conducting layers 310 d, 310 eand the dielectric layer 320 d of sub-composite structure 402 with theunaltered etch resist 400 removed, according to certain embodiments. Theunaltered etch resist 400 (FIGS. 4-6) may be removed by methods wellknown in the art, thereby exposing the conducting layer 310 d.

FIG. 8 is an illustration depicting the conducting layers 310 d, 310 eand the dielectric layer 320 d of sub-composite structure 402 with theplating resist 870 deposited within the clearance 600, according tocertain embodiments.

For example, a plating resist can be deposited into a clearance usingprinting, stencil printing, needle dispensing, etc. The plating resistcan be a hydrophobic insulating material that is resistant to thedeposition of a catalytic species capable of catalyzing an electrolessmetal deposition. The plating resist can also be a material that resistsdeposition of other “seed” deposits such as colloidal graphite.

The plating resist can be deposited so as to be flush or higher than theetched clearance layer. The plating resist can be a paste or viscousliquid. Some non-limiting examples of plating resists are siliconeresins, polyethylene resins, fluorocarbon resins, polyurethane resins,and acrylic resins. Such insulating hydrophobic resinous material can beused alone or in a combined composition with other resinous materials inamounts sufficient to maintain hydrophobic properties in the combinedcomposition.

After depositing the plating resist, the plating resist is cured usingappropriate methods. The sub-composite structure 402 with plating resist870 in place can now be laminated to the rest of the multilayer PCBstackup using techniques well known in the art. Multiple sub-compositestructures (cores) with selectively deposited plating resist areas invarying locations can be laminated to form a PCB stackup. Through-holesare drilled through the PCB stackup through conductive layers,dielectric layers and through the plating resist.

Thus, the PCB panel has multiple through-holes that can then be platedsimultaneously by placing the panel into a seed bath, followed byimmersion in an electroless copper bath. A non-limiting example of aseed bath is copper palladium colloid. An example for surface platingcan be found in U.S. Pat. No. 4,668,532. The electroless copper providesthe initial conductivity path to allow for additional electrolyticcopper plating of the barrel of each through-hole in the panel. The seedchemistry (electroless copper) will deposit on the surface of thethrough-hole wall, but will not deposit effectively on areas of the wallwith the plating resist. A small amount of electroless copper maydeposit on the plating resist but such an amount can be removed with apost processing step known in the art. For example, any small amounts ofelectroless copper that may be deposited on the plating resist can beremoved by contacting the affected areas with a chelating agent in analkaline solution for a time period sufficient to remove essentially allof said catalytic species from the hydrophobic plating resist. The panelwill then follow known processes either for panel plating or patternplating. For example, electrolytic or electroless plating can be used.In other words, the interior walls of the through-holes are contactedwith a metal deposition solution to metallize only the exposed catalyticareas of the walls not protected by the hydrophobic plating resist.

Plating of conductive material in the via structure will build whereverthere is seed material. Similarly, no plating of conductive materialwill form where there is plating resist. Thus, the areas that are voidof plated conductive material in the via structure effectively partitionthe via into electrically isolated sections. By strategically placingplating resist in certain locations and on certain layers of a PCBstackup, multiple electrically isolated portions in via structures canbe formed, simultaneously.

Thus, the above method can be used to configure the via structure intomultiple electrically isolated segments. Each such segment providesinterconnect paths to appropriate layers within the PCB. Suchpartitioned vias can be subsequently filled with an insulating materiallike epoxy or other insulating or resistive polymer for improvedreliability or increased functionality. Therefore, costly, error prone,and time intensive backdrilling may be avoided. Similarly, referringback to FIG. 3. the use of the plating resist 370 avoids possible damageto the PCB 300 which may result by backdrilling. A further advantage isthat, whereas backdrilling is typically controllable to a depthtolerance of +/−5 mils, a controllable depth tolerance of +/−1 mils orbetter may be achieved by the systems and methods described herein. As aresult, the consistency between the plating resist 370, the dielectriclayers 320 b, and the conducting layer 310 c may be held to a tighterstandard deviation as compared to backdrilling.

According to certain embodiments, a thicker resist deposit may bepreferred. In such a case, the sub-composite structure or core ismechanically drilled with through-holes corresponding to areas wherepartitioned via structures are desired in the resulting PCB stackup. Thethickness of the sub-composite structure can range from about 1-50 mils.Thus, a thicker deposit of plating resist can be produced. Thethrough-holes are filled with plating resist using specialized holefilling equipment, stenciling or screen printing. Such a process isknown as hole-plugging or via-filling. The plating resist is then curedusing an appropriate process. A planarizing or scrubbing operation maybe employed to remove any excess plating resist from the surface of thesub-composite structure. The sub-composite structure can be processedusing standard PCB procedures to form circuit images. It is to be notedthat the through-holes can be filled with plating resist before or afterforming circuit images. The sub-composite structure can then belaminated into a multilayer PCB stackup and the process can continue asdescribed above for electroless seeding and subsequent plating of theinterior walls of the one or more via structures in the PCB stackup.According to certain embodiments, the partitioned via is filled with anelectrically insulating material, ohmically resistive paste or voltageswitchable dielectric material to improve reliability or functionality.In the case of using voltage switchable dielectric material,programmable circuit routing in PCBs can be made. Further, the voltageswitchable dielectric material can provide transient protection. Theterm “transient” as used herein encompasses not only electrostaticdischarge events but any phenomena, of short duration, that directly orindirectly induces voltages and currents into a printed circuit boardand where the amplitudes of such voltages and currents are high enoughto cause degradation or failure of the electronic components on theprinted circuit board.

FIG. 9 is an illustration depicting a PCB stackup with a partitionedplated via structure formed using a thicker layer of plating resist,according to certain embodiments. FIG. 9 shows a PCB 900 that includesconducting layers 910 a-910 f separated by dielectric layers 920 a-920f. The plated via structure 930 is plated with a seed conductivematerial 990 and a further coating of conductive material 992. Theplated via 930 is effectively partitioned into a plurality ofelectrically isolated portions (930 a, and 930 b) by selectivelydepositing plating resist in a sub-composite structure used for makingthe PCB stackup.

FIG. 9 shows that the partitioned plated via allows an electrical signal960 to transmit from one trace 940 on a first conducting layer 910 a toanother trace 950 on a second conducting layer 910 b of the PCB 900 bytraversing the isolated portion 930 a of the via 930 without signalintegrity being degraded through interference caused by portion 930 b.The conductive material 992 of the plated via structure 930 is themedium through which the electrical signal 960 travels from the firstconducting layer 910 a of the PCB 900 to the second conducting layer 910b. Similarly, the isolated portion 930 b of the via 930 allows anotherelectrical signal 962 to transmit to trace 980 without interfering withthe signal 960. The plating resist 970 prevents the deposition of theconductive material 990 and 992 within the via structure 930 at theconducting layers 910 c and 910 d. As a result, via 930 is effectivelypartitioned into the electrically isolated portions 930 a, and 930 b.

The PCB 900 can have any number of conducting layers and dielectriclayers. FIG. 9 only shows six layers of conducting layers 910 a-910 fand six layers of dielectric layers 920 a-920 f for the sake ofsimplicity. Each of the conducting layers 910 a-910 f may comprise apartial or full layer such as a power or ground layer, and may comprisea layer of circuit traces, or may comprise a layer with both circuittraces and a partial layer such as a ground layer. A non-limitingexample of the conducting layers 910 a-910 f is copper and somenon-limiting examples of dielectric layers 920 a-920 f are epoxy glass,polyimide glass, ceramic hydrocarbon, polyimide film, Teflon film, resinimpregnated matte material, Kevlar, paper, resin dielectrics withdispersed nano-powders.

According to certain embodiments, plating resist is selectivelydeposited in a clearance formed in a conducting layer and an adjacentdielectric layer of a sub-composite structure. In such a case, thesub-composite structure can be mechanically or laser drilled to form ablind hole. The blind hole starts at one conductive layer of thesub-composite structure, proceeds through the dielectric layer andterminates on another conductive layer of the sub-composite structure.However, the depth of the blind hole can be drilled to any depth shortof reaching the conductive layer of the sub-composite structure. Platingresist is then deposited into the blind hole using a squeegeeing,stenciling, or screen printing operation, for example. The resist isthen cured. A planarizing or scrubbing operation may be employed toremove resist from the open end of the blind hole. The sub-compositestructure can be processed using standard PCB procedures to form circuitimages. It is to be noted that the plating resist can be depositedbefore or after forming circuit images. The sub-composite structure canthen be laminated into a multilayer PCB stackup and the process cancontinue as described above for electroless seeding and subsequentplating of the interior walls of the via structure. The advantage insuch a via structure is that the plating resist does not come out of theblind end of the hole and a connection can be made to the undrilledconductive layer of the sub-composite structure (core). According tocertain embodiments, the partitioned via is filled with an electricallyinsulating material, ohmically resistive paste or voltage switchabledielectric material to improve reliability or functionality. In the caseof using voltage switchable dielectric material, programmable circuitrouting in PCBs can be made. Further, the voltage switchable dielectricmaterial can provide transient protection.

FIG. 10 is an illustration depicting a PCB stackup with a partitionedplated via structure formed by selectively depositing plating resist ina clearance formed in a conducting layer and an adjacent dielectriclayer of a sub-composite structure, according to certain embodiments.FIG. 10 shows a PCB 1000 that includes conducting layers 1010 a-1010 fseparated by dielectric layers 1020 a-1020 f. The plated via structure1030 is plated with a seed conductive material 1090 and a furthercoating of conductive material 1092. The plated via 1030 is effectivelypartitioned into a plurality of electrically isolated portions (1030 a,and 1030 b) by selectively depositing plating resist in a sub-compositestructure used for making the PCB stackup.

FIG. 10 shows that the partitioned plated via allows an electricalsignal 1060 to transmit from one trace 1040 on a first conducting layer1010 a to another trace 1050 on a different conducting layer 1010 c ofthe PCB 1000 by traversing the isolated portion 1030 a of the via 1030without signal integrity being degraded through interference caused byportion 1030 b. The conductive material 1092 of the plated via structure1030 is the medium through which the electrical signal 1060 travels fromthe first conducting layer 1010 a of the PCB 1000 to the anotherconducting layer 1010 c. Similarly, the isolated portion 1030 b of thevia 1030 allows another electrical signal 1062 to transmit to trace 1080without interfering with the signal 1060. The plating resist 1070prevents the deposition of the conductive material 1090 and 1092 withinthe via structure 1030 at the conducting layer 1010 d and the dielectriclayer 1020 c. As a result, via 1030 is effectively partitioned into theelectrically isolated portions 1030 a, and 1030 b.

The PCB 1000 can have any number of conducting layers and dielectriclayers. FIG. 10 only shows six layers of conducting layers 1010 a-1010 fand six layers of dielectric layers 1020 a-1020 f for the sake ofsimplicity. Each of the conducting layers 1010 a-1010 f may comprise apartial or full layer such as a power or ground layer, and may comprisea layer of circuit traces, or may comprise a layer with both circuittraces and a partial layer such as a ground layer. A non-limitingexample of the conducting layers 1010 a-1010 f is copper and somenon-limiting examples of dielectric layers 1020 a-1020 f are epoxyglass, polyimide glass, ceramic hydrocarbon, polyimide film, Teflonfilm, resin impregnated matte material, Kevlar, paper, resin dielectricswith dispersed nano-powders.

According to certain embodiments, plating resist is selectivelydeposited on the surface of a sub-composite structure on the exposeddielectric on the surface that is coplanar with the top conducting layerof the sub-composite structure. In such a case, the plating resist isdeposited onto an etched surface of a sub-composite core on the exposeddielectric. The plating resist is deposited onto the dielectric usingscreen printing, stenciling, needle depositing or other methods know inthe art. The thickness of the deposit of plating resist can be adjustedto a range up to 5 mils thick. The deposit of plating resist can be anyshape but typically would be round or square in geometry. Afterdeposition, the resist is cured using appropriate process. Thesub-composite structure can be processed using standard PCB proceduresto form circuit images. It is to be noted that the plating resist can bedeposited before or after forming circuit images. The sub-compositestructure can then be laminated into a multilayer PCB stackup and theprocess can continue as described above for electroless seeding andsubsequent plating of the interior walls of the via structure. Accordingto certain embodiments, the partitioned via is filled with anelectrically insulating material, ohmically resistive paste or voltageswitchable dielectric material to improve reliability or functionality.In the case of using voltage switchable dielectric material,programmable circuit routing in PCBs can be made. Further, the voltageswitchable dielectric material can provide transient protection.

FIG. 11 is an illustration depicting a PCB stackup with a partitionedplated via structure formed by selectively depositing plating resist onthe surface of a sub-composite structure on the exposed dielectric,according to certain embodiments. FIG. 11 shows a PCB 1100 that includesconducting layers 1110 a-1110 e separated by dielectric layers 1120a-1120 e. The plated via structure 1130 is plated with a seed conductivematerial 1190 and a further coating of conductive material 1192. Theplated via 1130 is effectively partitioned into a plurality ofelectrically isolated portions (1130 a, and 1130 b) by selectivelydepositing plating resist in a sub-composite structure used for makingthe PCB stackup.

FIG. 11 shows that the partitioned plated via allows an electricalsignal 1160 to transmit from one trace 1140 on a first conducting layer1110 a to another trace 1150 on a different conducting layer 1110 c ofthe PCB 1100 by traversing the isolated portion 1130 a of the via 1130without signal integrity being degraded through interference caused byportion 1130 b. The conductive material 1192 of the plated via structure1130 is the medium through which the electrical signal 1160 travels fromthe first conducting layer 1110 a of the PCB 1100 to the anotherconducting layer 1110 c. Similarly, the isolated portion 1130 b of thevia 1130 allows another electrical signal 1162 to transmit to trace 1180without interfering with the signal 1160. The plating resist 1170prevents the deposition of the conductive material 1190 and 1192 withinthe via structure 1130 at an area between the conducting layer 1110 cand another conductive layer 1110 e. As a result, via 1130 iseffectively partitioned into the electrically isolated portions 1130 a,and 1130 b. The plated via structure 1130 may be of any shape.

The PCB 1100 can have any number of conducting layers and dielectriclayers. FIG. 11 only shows five layers of conducting layers 1110 a-1110e and five layers of dielectric layers 1120 a-1120 e for the sake ofsimplicity. Each of the conducting layers 1110 a-1110 e may comprise apartial or full layer such as a power or ground layer, and may comprisea layer of circuit traces, or may comprise a layer with both circuittraces and a partial layer such as a ground layer. A non-limitingexample of the conducting layers 1110 a-1110 e is copper and somenon-limiting examples of dielectric layers 1120 a-1120 e are epoxyglass, polyimide glass, ceramic hydrocarbon, polyimide film, Teflonfilm, resin impregnated matte material, Kevlar, paper, resin dielectricswith dispersed nano-powders.

According to certain embodiments, plating resist is selectivelydeposited on the surface of a sub-composite structure on a conductiveregion or conductive pad on the surface of the sub-composite structure.The conductive region could be patterned to be a plane or could be anindividual pad or feature. In the case of a pad or feature, the platingresist may overlap the pad. The plating resist is deposited onto theconductive region using screen printing, stenciling, needle depositingor other methods know in the art. The deposit of plating resist can beany shape but typically would be round or square in geometry. Afterdeposition, the resist is cured using appropriate process. Thesub-composite structure can be processed using standard PCB proceduresto form circuit images. It is to be noted that the plating resist can bedeposited before or after forming circuit images. The sub-compositestructure can then be laminated into a multilayer PCB stackup and theprocess can continue as described above for electroless seeding andsubsequent plating of the interior walls of the via structure. Accordingto certain embodiments, the partitioned via is filled with anelectrically insulating material, ohmically resistive paste or voltageswitchable dielectric material to improve reliability or functionality.In the case of using voltage switchable dielectric material,programmable circuit routing in PCBs can be made. Further, the voltageswitchable dielectric material can provide transient protection.

FIG. 12 is an illustration depicting a PCB stackup with a partitionedplated via structure formed by selectively depositing plating resist ona conductive region or conductive pad on the surface of thesub-composite structure, according to certain embodiments. FIG. 12 showsa PCB 1200 that includes conducting layers 1210 a-1210 e separated bydielectric layers 1220 a-1220 e. The plated via structure 1230 is platedwith a seed conductive material 1290 and a further coating of conductivematerial 1292. The plated via 1230 is effectively partitioned into aplurality of electrically isolated portions (1230 a, and 1230 b) byselectively depositing plating resist in a sub-composite structure usedfor making the PCB stackup.

FIG. 12 shows that the partitioned plated via allows an electricalsignal 1260 to transmit from one trace 1240 on a first conducting layer1210 a to another trace 1250 on the conducting pad 1210 d of the PCB1200 by traversing the isolated portion 1230 a of the via 1230 withoutsignal integrity being degraded through interference caused by portion1230 b. The conductive material 1292 of the plated via structure 1230 isthe medium through which the electrical signal 1260 travels from thefirst conducting layer 1210 a of the PCB 1200 to the conducting pad 1210d. Similarly, the isolated portion 1230 b of the via 1230 allows anotherelectrical signal 1262 to transmit to trace 1280 without interferingwith the signal 1260. The plating resist 1270 prevents the deposition ofthe conductive material 1290 and 1292 within the via structure 1230 atan area between the conducting layer 1210 e and the conducting pad 1210d. As a result, via 1230 is effectively partitioned into theelectrically isolated portions 1230 a, and 1230 b. The plated viastructure 1230 may be of any shape.

The PCB 1200 can have any number of conducting layers and dielectriclayers. FIG. 12 only shows five layers of conducting layers 1210 a-1210e and five layers of dielectric layers 1120 a-1120 e for the sake ofsimplicity. Each of the conducting layers 1210 a-1210 e may comprise apartial or full layer such as a power or ground layer, and may comprisea layer of circuit traces, or may comprise a layer with both circuittraces and a partial layer such as a ground layer. A non-limitingexample of the conducting layers 1210 a-1210 f is copper and somenon-limiting examples of dielectric layers 1220 a-1220 e are epoxyglass, polyimide glass, ceramic hydrocarbon, polyimide film, Teflonfilm, resin impregnated matte material, Kevlar, paper, resin dielectricswith dispersed nano-powders.

Due to the selective nature of the plating resist deposition andsimultaneous plating of the vias resulting in partitioned sections, viascan be subdivided into multiple sections each capable of carryingsignals without disturbing signals in other sections. To do soeffectively, a computer program is advantageous to use when designing aPCB layout. For example, the computer program would be patched to anECAD software such as Cadence Allegro™ or Mentor Expedition™ orSupermax™. The computer program can also run as a stand alone softwaremodule, which would import data from an ECAD system, partition the vias,then output appropriate files back to the ECAD or Computer AidedManufacturing (CAM) system. Such software can also output files to beused for programming manufacturing equipment to drill appropriate holesin selected cores and/or generate art work to manufacture stencils forselective deposition of the plating resist. Thus, by determining thelocations of the plating resist and location of the resultingpartitioned vias, a PCB design can be optimized to increase routingdensity and improve integrity. In the case of a pre-existing design of aPCB layout, the computer program can be used to identify locations forselective depositions of plating resist in locations that correlate tolocations for backdrilling, for example.

In the foregoing specification, embodiments of the invention have beendescribed with reference to numerous specific details that may vary fromimplementation to implementation. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense. The invention is intended to be as broad as the appended claims,including all equivalents thereto.

1. A multilayer PCB, said multilayer PCB comprising: at least onesub-composite structure having: a dielectric layer sandwiched between aconductive pad and a conductive layer; and a selective deposit ofplating resist deposited on said conductive pad; a through-hole drilledthrough said multilayer PCB and passing through said layer of platingresist, wherein an interior surface of said through-hole is plated withconductive material in areas that are devoid of said plating resist toform a partitioned via structure through said multilayer PCB.
 2. Asub-composite structure for use with a multilayer PCB, saidsub-composite structure comprising: a dielectric layer sandwichedbetween a conductive pad and a conductive layer, and a selective depositof plating resist deposited on said conductive pad.